Plasma arc torches are widely used in the high temperature processing (e.g., cutting, welding, and marking) of metallic materials. As shown in FIG. 1A, a plasma arc torch generally includes a torch body 1, an electrode 2 mounted within the body, an insert 3 disposed within a bore of the electrode 2, a nozzle 4 with a central exit orifice, a shield 5, electrical connections (not shown), passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply (not shown). The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. A gas can be non-reactive, e.g. nitrogen or argon, or reactive, e.g. oxygen or air.
In the process of plasma arc cutting or marking a metallic workpiece, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). The pilot arc ionizes gas that passes through the nozzle exit orifice. After the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc then transfers from the nozzle to the workpiece. Generally the torch is operated in this transferred plasma arc mode, which is characterized by the conductive flow of ionized gas from the electrode to the workpiece, for the cutting, welding, or marking the workpiece.
In a plasma arc torch using a reactive plasma gas, it is known to use a copper electrode with an insert of high thermionic emissivity material. FIGS. 1B-1D illustrate a known method for inserting and securing an insert into the bore of an electrode. FIG. 1B illustrates an insert 10 being pressed 15 into a bore in the end of an electrode body 12. FIG. 1C illustrates the secured insert 11 pressed 15 flush with the end surface 19 of the electrode body 12, and presents a diagrammatic representation of the resultant lateral forces securing the insert 11 in the electrode body 12. These resultant forces are thought to be greater near the exposed end of the insert due to surface friction from the expanding insert. When assembling inserts of known configuration into straight-walled bores, the insert tends to expand radially more near the top of the bore than at the closed end of the bore, tending to produce a wedge shape. A radial bulge sometimes forms near the open end of the bore 14. This tapered bulge is not unexpected since the insert is pressed only from the exposed end. During pressing, once the bore is essentially filled with the insert and can no longer accept more insert material, any remaining insert material pressed in from the open end of the bore tends to form a bulge at the open end of the bore where the hoop strength of the electrode body is not as great. The resulting configuration initially secures the insert, but any movement of the insert towards the opening of the bore significantly reduces the surface contact and retention force of the insert. FIG. 1D illustrates a secured insert 17 in a through-hole configuration of the bore, where 19 is a volume defined by the inner surface of the electrode body 16. The insert 17 is pressed from both sides in this configuration, where the force 18 can be supplied from an anvil or mandrel pressed into the volume 19, for installation of the insert. Electrode bodies of the through-hole type 19 are also known to have linear-tapered walls, i.e., straight walls at an angle with a central longitudinal axis, with linear-tapered inserts shaped to match.
The insert has an exterior, or exposed, end face, which defines an emissive surface area. The exterior surface of the insert is generally planar, and is manufactured to be coplanar with the end face of the electrode. The end face of the electrode is typically planar, although it can have exterior curved surfaces, e.g., edges. It is known to make the insert of hafnium or zirconium. They generally have a cylindrical shape. Insert materials (e.g., hafnium) can be expensive.
During the operation of plasma arc torch electrodes, torch conditions such as temperature gradients and dynamics work to reduce the retention force holding the insert in place and either allow the insert to move in the bore or to fall completely out of the bore, thereby reducing the service life of the electrode or causing it to completely fail. The movement of the insert also indicates that the insert to electrode interface has degraded, which reduces the thermal and electrical conductivity of the interface and thereby the service life of the electrode as well. In addition, insert materials (e.g., hafnium) are poor thermal conductors for the removal of heat produced by the plasma arc, which can produce temperatures in excess of 10,000 degrees C. Insufficient removal of heat resulting from these high temperatures can result in a decrease in the service life of the electrode.
What is needed is an electrode with improved retention of the insert within the bore. A first object of the invention is to provide an electrode with improved retention of an insert, increasing the thermal conductivity of the interface between insert and electrode, and the efficiency and service life of the electrode. It is another object of the invention to provide an electrode with an insert configuration that improves the cooling, and therefore the service life, of the insert. It is yet another object of the invention to provide an electrode with an insert configuration that minimizes the amount of insert material required, thereby reducing the cost of the electrode while at the same time not lessening the efficiency and service life of the electrode. Yet another object of the invention is to provide an electrode with a longer service life.